To improve the impact properties of polypropylene homopolymers (and random copolymers), an elastomeric component is typically added, either by way of the production of an in-reactor blend of a propylene polymer and an elastomeric component (an impact propylene copolymer) or by way of compounding of a propylene polymer and an elastomeric component. In the former method the propylene polymer and the elastomeric component are produced in one or more reactors of the same process.
Both of these methods of improving the impact of polypropylene do not significantly contribute to increasing the melt strength of the resulting polypropylene impact copolymer.
And although both methods result in a propylene polymer with improved impact properties (when formed into articles), these impact properties often show an imbalance between the notched impact tested parallel to the polymer injection flow direction of the injection molded product, and the notched impact tested perpendicular to the polymer injection flow direction of the injection molded product. The notched impact tested on specimens cut perpendicular to the direction of orientation (that is perpendicular to the polymer injection flow direction), with the notch in the direction of orientation (parallel to the polymer injection flow direction), is typically much lower. The polymer injection flow direction is the direction or line along which the polymer is introduced into a mold or the direction or line along which the polymer is extruded. Polymers such as propylene polymers will tend to orient themselves along this line of direction.
What is desired is a polymer resin, which provides directionally balanced impact properties for articles made from the resin, while simultaneously having increased melt strength. Additionally, it would be desirable to provide a polymer resin, which is easy to fabricate into articles and exhibits a ductile-to-brittle transition temperature of 0xc2x0 C. or lower.
As used herein, the following terms shall have the following meanings:
xe2x80x9cCoupling agentxe2x80x9d means a chemical compound that contains at least two reactive groups that are each capable of forming a carbene or nitrene group that are capable of inserting into the carbon hydrogen bonds of CH, CH2, or CH3 groups, both aliphatic and aromatic, of a polymer chain. The reactive groups together can couple polymer chains. It may be necessary to activate a coupling agent with heat, sonic energy, radiation or other chemical activating energy, for the coupling agent to be effective for coupling polymer chains. Examples of chemical compounds that contain a reactive group capable of forming a carbene group include, for example, diazo alkanes, geminally-substututed methylene groups, and metallocarbenes. Examples of chemical compounds that contain reactive groups capable of forming nitrene groups, include, but are not limited to, for example, phosphazene azides, sulfonyl azides, formyl azides, and azides.
xe2x80x9cImpact propylene copolymersxe2x80x9d are commercially available and are well known within the skill in the art, for instance, as described by E. P. Moore, Jr in Polypropylene Handbook, Hanser Publishers, 1996, page 220-221 and U.S. Pat. Nos. 3,893,989 and 4,113,802. The term xe2x80x9cimpact propylene copolymerxe2x80x9d is used herein to refer to heterophasic propylene copolymers where polypropylene is the continuous phase and an elastomeric phase is dispersed therein. Those of skill in the art recognize that this elastomeric phase may also contain crystalline regions, which for purposes of the current invention are considered part of the elastomeric phase. The impact propylene copolymers result from an in-reactor process rather than physical blending. Usually the impact propylene copolymers are formed in a dual or multi-stage process, which optionally involves a single reactor with at least two process stages taking place therein, or optionally multiple reactors.
xe2x80x9cImpact propertiesxe2x80x9d refer to properties of articles such as impact strength, which is measured by any means within the skill in the art, for instance, Izod impact energy as measured in accordance with ASTM D 256, MTS Peak Impact Energy (dart impact) as measured in accordance with ASTM D 3763-93, and MTS total Impact Energy as measured in accordance with ASTM D-3763. The ductile-to-brittle transition temperature (DBTT) is also an impact property of an article made from a polymer. The ductile-to-brittle transition temperature defines, for a given set of conditions, the temperature at which an object transitions from a predominantly ductile mode of failure to a predominantly brittle mode of failure. The ductile-tobrittle transition temperature can be calculated using techniques known to one of skill in the art.
The invention includes a composition comprising a coupled impact propylene copolymer. The coupled impact propylene copolymer is formed by the reaction of a coupling agent, such as a bis(sulfonyl azide), with an impact propylene copolymer. Advantageously, the impact propylene copolymers used for the invention have at least about 5 weight percent, preferably at least about 9 weight percent, more preferably at least about 13 weight percent, of an elastomeric phase based on the total weight of the impact propylene copolymer. Preferably, the elastomeric phase is less than about 45 weight percent, more preferably less than about 40 weight percent, most preferably less than about 35 weight percent, of the total weight of the impact propylene copolymer.
When the continuous phase of the impact propylene copolymer is a homopolymer propylene polymer and the elastomeric phase is comprised of a copolymer or terpolymer containing monomer units derived from ethylene, the impact propylene copolymer preferably contains at least about 5 weight percent, more preferably at least about 7 weight percent, most preferably at least about 9 weight percent xe2x80x94CH2CH2xe2x80x94 units derived from ethylene monomer based on the total weight of the impact propylene copolymer. Preferably, such an impact propylene copolymer contains less than about 30 weight percent, more preferably less than about 25 weight percent, most preferably less than about 20 weight percent xe2x80x94CH2CH2xe2x80x94 units derived from ethylene monomer based on the total weight of the impact propylene copolymer.
The invention also includes a method for coupling an impact propylene copolymer using a coupling agent, such as a bis(sulfonyl azide). The method improves the impact properties of the impact propylene copolymer and also increases the melt strength of the resulting coupled impact propylene copolymer resin to a level of at least about 1.25 times, preferably at least about 1.5 times, that of a comparable noncoupled impact propylene copolymer. All values for xe2x80x9cmelt strengthxe2x80x9d are determined by the method as set forth in the Examples. A comparable noncoupled impact propylene copolymer is the same polymer used to make the coupled impact propylene copolymer, but has not been coupled. Preferably, the coupled impact propylene copolymer resins have a melt strength of at least about 8, more preferably a melt strength of at least about 15 cN, further more preferably a melt strength of at least about 30 cN, most preferably a melt strength of at least about 50 cN and in some instances a melt strength of at least about 60 cN.
Examples of impact properties that are improved in the coupled impact propylene copolymer of the invention compared to the comparable noncoupled impact propylene copolymers, include: higher impact strength at low temperatures as exhibited by articles formed from the coupled impact propylene copolymer and an improvement in the ductile-to-brittle transition temperature, which is reduced in articles formed from the coupled impact propylene copolymer of the invention.
Additionally, in all aspects, articles formed from the coupled impact propylene copolymer of the invention exhibit an improvement in the directional balance of the impact properties as compared to articles formed from a comparable noncoupled impact propylene copolymer. (i.e. a reduction of the imbalance between the impact properties as measured parallel to the polymer injection flow direction versus perpendicular to the polymer injection flow direction). This reduction in directional imbalance of the impact properties is indicated by a reduction in the difference between ductile-to-brittle transition temperatures measured parallel and perpendicular to the polymer injection flow direction for an article fabricated from the coupled impact propylene copolymer of the invention as compared with the difference between ductile-to-brittle transition temperatures measured parallel and perpendicular to the polymer injection flow direction for an article fabricated from a comparable noncoupled impact propylene copolymer.
In some aspects of the invention, the reduction in the DBTT for articles formed from the coupled impact propylene copolymer of the current invention versus articles formed from comparable noncoupled impact propylene copolymers (as calculated from the notched Izod values measured in accordance with ASTM D-256 with the notch perpendicular to the polymer injection flow direction) will be at least about 10xc2x0 C., preferably at least about 15xc2x0 C., more preferably at least about 20xc2x0 C. and in some instances at least about 25xc2x0 C.
In another aspect of the invention, articles formed from the coupled impact propylene copolymers of the current invention exhibit a DBTT as calculated from notched Izod values measured in accordance with ASTM D-256 with the notch perpendicular to the polymer injection flow direction, of about xe2x88x925xc2x0 C. or less, preferably of about xe2x88x9210xc2x0 C. or less, more preferably of about xe2x88x9215xc2x0 C. or less, most preferably xe2x88x9220xc2x0 C. or less.
For all the embodiments of the invention, the articles to be tested to determine DBTT should be edge-gated injection molded plaques that have a single polymer injection flow direction. For all embodiments, the Izod values are measured in accordance with ASTM D-256 and the values for DBTT are calculated in accordance with techniques known to one of ordinary skill in the art.
In a most preferred embodiment, articles made from the coupled impact propylene copolymers exhibit improvements in the directional balance of the impact properties together with improvements in the DBTT and a reduction of the DBTT as compared with comparable noncoupled impact propylene copolymers.
Further, articles formed from the coupled impact propylene copolymer resins of the present invention advantageously maintain the stiffness of the comparable noncoupled impact propylene copolymer.
The process to produce this improved impact propylene copolymer involves coupling of the impact propylene copolymer using a coupling agent. The coupling reaction is implemented via reactive extrusion or any other method which is capable of mixing the coupling agent with the impact propylene copolymer and adding sufficient energy to cause a coupling reaction between the coupling agent and the impact propylene copolymer. Preferably, the process is carried out in a single vessel such as a melt mixer or a polymer extruder, such as described in U.S. patent application SER. No. 09/133,576 filed Aug. 13, 1998 which claims the benefit of U.S. Provisional Application No. 60/057,713 filed Aug. 27, 1997, both of which are incorporated by reference herein in their entity. The term extruder is intended to include its broadest meaning and includes such devices as a device which extrudes pellets as well as an extruder which produces the extrudate for forming into films, blow molded articles, profile and sheet extruded articles, foams and other articles.
As discussed earlier, the impact propylene copolymers have a continuous phase, which is comprised of a propylene polymer, and an elastomeric phase. The propylene polymer of the continuous phase typically will be a homopolymer propylene polymer or a random propylene copolymer, more typically a homopolymer propylene polymer. The propylene polymer may be made using Ziegler-Natta catalyst, constrained geometry catalyst, metallocene catalyst, or any other suitable catalyst system. When the propylene polymer making up the continuous phase is a homopolymer propylene polymer, the crystallinity of the propylene polymer, as determined by differential scanning calorimetry, is preferably at least about 50 percent, more preferably at least about 55 percent, most preferably at least about 62 percent. The methods for determining percent crystallinity using a differential scanning calorimetry are know to one of skill in the art.
The elastomeric phase may be made using constrained geometry catalyst, Ziegler-Natta catalyst, metallocene catalyst, or any other suitable catalyst.
The preferred coupling agent is a poly(sulfonyl azide), more preferably a bis(sulfonyl azide). Examples of poly(sulfonyl azides) useful for the invention are described in WO 99/10424. Poly(sulfonyl)azides include such compounds as 1, 5-pentane bis(sulfonyl azide), 1,8-octane bis(sulfonyl azide), 1,10-decane bis(sulfonyl azide), 1,10-octadecane bis(sulfonyl azide), 1-octyl-2,4,6-benzene tris(sulfonyl azide), 4,4xe2x80x2-diphenyl ether bis(sulfonyl azide), 1,6-bis(4xe2x80x2-sulfonazidophenyl)hexane, 2,7-naphthalene bis(sulfonyl azide), and mixed sulfonyl azides of chlorinated aliphatic hydrocarbons containing an average of from 1 to 8 chlorine atoms and from 2 to 5 sulfonyl azide groups per molecule, and mixtures thereof.
Preferred poly(sulfonyl azide)s include oxy-bis(4-sulfonylazidobenzene), 2,7-naphthalene bis(sulfonyl azido), 4,4xe2x80x2-bis(sulfonyl azido)biphenyl, 4,4xe2x80x2-diphenyl ether bis(sulfonyl azide) and bis(4-sulfonyl azidophenyl)methane, and mixtures thereof.
Sulfonyl azides are commercially available or are conveniently prepared by the reaction of sodium azide with the corresponding sulfonyl chloride, although oxidation of sulfonyl hydrazines with various reagents (nitrous acid, dinitrogen tetroxide, nitrosonium tetrafluoroborate) has been used.
In the practice of the invention preferably sufficient coupling agent is used to cause articles made from the resulting coupled impact propylene copolymer to exhibit a reduction in the difference between ductile-to-brittle transition temperatures measured parallel and perpendicular to the polymer injection flow direction as compared with the difference between ductile-to-brittle transition temperatures measured parallel and perpendicular to the polymer injection flow direction for an article fabricated from a comparable noncoupled impact propylene copolymer. When a bis(sulfonyl azide) is used for the coupling agent, preferably at least about 100 ppm of azide is used for coupling the impact propylene copolymer, based on the total weight of the impact propylene copolymer, more preferably at least about 150 ppm of azide, most preferably at least about 200 ppm of azide is used. In some instances, such as where a large reduction in the ductile-to-brittle transition temperature is desirable as compared with the base comparable noncoupled impact propylene copolymer, at least about 300 ppm of bis(sulfonyl azide), preferably at least about 450 ppm of bis(sulfonyl azide) based on the total weight of the impact propylene copolymer is used for coupling the impact propylene copolymer. It is important in choosing the impact propylene copolymer to be coupled, that a polymer is chosen that has a high enough melt flow rate, so that after coupling with the desired amount of coupling agent, the coupled impact propylene copolymer will have a sufficiently high melt flow rate to be readily processed.
In a preferred aspect of the invention the coupled impact propylene copolymer of the invention can be characterized by the following formula:
X=[(Axe2x88x92C)/(Bxe2x88x92D)]xe2x89xa60.75 and Yxe2x89xa71.25 and Axe2x89xa6Bxe2x88x9210, in which
A=the ductile-to-brittle transition temperature calculated from notched Izod values (Measured in accordance with ASTM D-256) measured with the notch perpendicular to the polymer injection flow direction for an article made from the coupled impact propylene copolymer resin.
B=the ductile-to-brittle transition temperature calculated from notched Izod values (Measured in accordance with ASTM D-256) measured with the notch perpendicular to the polymer injection flow direction for an article made from the comparable noncoupled impact propylene copolymer resin.
C=the ductile-to-brittle transition temperature calculated from notched Izod values (Measured in accordance with ASTM D-256) measured with the notch parallel to the polymer injection flow direction for an article made from the coupled impact propylene copolymer resin.
D=the ductile-to-brittle transition temperature calculated from notched Izod (Measured in accordance with ASTM D-256) measured with the notch parallel to the polymer injection flow direction for an article made from the comparable noncoupled impact propylene copolymer resin.
Y=the ratio of the melt strength of the coupled impact propylene copolymer resin to the melt strength of the comparable noncoupled impact propylene copolymer resin. Preferably, in this aspect, Y is at least about 1.5; more preferably, Y is at least about 2; most preferably, Y is at least about 5; and in some instances, Y is at least about 10.
Preferably, in this aspect, X is less than about 0.5, more preferably less than about 0.33, most preferably less than about 0.25.
For purposes of this invention: the value for A above may sometimes be referred to as the xe2x80x9cparallel ductile-to-brittle transition temperaturexe2x80x9d for an article made from the coupled impact propylene copolymer; the value for B above may sometimes be referred to as the xe2x80x9cparallel ductile-to-brittle transition temperaturexe2x80x9d for an article made from the comparable noncoupled impact propylene copolymer; the value for C above may sometimes be referred to as the xe2x80x9cperpendicular ductile-to-brittle transition temperaturexe2x80x9d for an article made from the coupled impact propylene copolymer; and the value for D above may sometimes be referred to as the xe2x80x9cperpendicular ductile-tobrittle transition temperaturexe2x80x9d for an article made from the comparable noncoupled impact propylene copolymer.